Buoyancy pressures and hydrocarbon presence

	Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Formation fluid pressure and its application
Author	Edward A. Beaumont, Forrest Fiedler
Link	Web page
Store	AAPG Store

Buoyancy pressures caused by hydrocarbon columns can be recognized by comparing hydrostatic pressure gradients with formation pressures. Pressures exceeding expected hydrostatic pressures could be due to the presence of hydrocarbon columns.

Two items are critical for detecting buoyancy pressure in a well:

Accurate static water plot for the well (See Constructing a hydrostatic pressure-depth plot)
Reliable formation fluid pressure measurements (See Methods for obtaining formation fluid pressures)

Procedure[edit]

The list below outlines a procedure for using pressures to detect the presence of a hydrocarbon column in a formation.

Make a hydrostatic pressure-depth plot through the interval of interest.
Plot the pressure(s) measured from the interval of interest.
If the measured formation pressures are greater than the hydrostatic pressure, then the formation may contain a hydrocarbon column.
Check to see if anomalous pressures make geological sense.
- Example: Measured fluid pressure is pressure::250 psi over the static water pressure. The formation is believed to contain 30°API gravity oil, and the total vertical trap closure is length::500 ft.
- Solution: If the 250-psi pressure is due to the presence of a hydrocarbon column, then a column of depth::2500 ft of 30°API gravity oil would have to be present in the trap (assuming a freshwater gradient). Vertical trap closure is only length::500 ft; therefore, the measured formation pressure does not match the geology and is probably wrong.

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Buoyancy pressures and hydrocarbon presence